Concrete is a brittle material. Concrete structures rely largely on the deformation and yielding of the tensile reinforcement to satisfy the ductility demand. The widespread application of high strength steel reinforcement in concrete structures has a significant drawback from a ductility point of view due to a lower degree of strain hardening and smaller ultimate elongation of the high strength steel. The application of fiber reinforced polymer (FRP) reinforcement encounters a similar problem, as FRPs have a low strain capacity and linear elastic stress-strain behavior up to rupture without yielding. The ductility of concrete members reinforced with non-ductile bars, especially FRP reinforced concrete (RC) members, has been a major concern in the studies of reinforced concrete structures in recent years.
Conventional RC members reinforced with ductile bars also have ductility problems when the failure is caused by the compressive crushing of concrete in which the tensile reinforcement does not yield. This occurs in over-reinforced RC beams and RC columns with a high axial load level. In this case the ductility and deformability of RC members are significantly reduced, although significant confinement to concrete can partially offset this reduction. The more the tensile reinforcement in an RC beam, the less the tensile reinforcement deforms and hence the lower the deformability and ductility of the member. Similarly, the higher the axial load level in an RC column, the lower the ductility. Furthermore, the use of more brittle high strength concrete (HSC), which has been increasing in a fast rate over the last two decades, has a similar detrimental side-effect on the ductility of reinforced concrete members especially for concrete columns.
Ductility of structures is important to ensure large deformation and give sufficient warning while maintaining an adequate load carrying capacity before structural failure, so that total collapse may be prevented and lives saved. Ductility is also the basis of modern structural design approaches (e.g. moment redistribution). In seismic design, in particular, ductility becomes an extremely important consideration. The issue of ductility and methods of increasing ductility is one of the most active areas in the study of concrete structures. There are a number of existing approaches used to improve the structural ductility of FRP reinforced concrete members, some of them are equally applicable to steel reinforced concrete members:
Providing confinement to concrete. Confinement increases ductility/deformability of concrete, however, this method cannot avoid the rupture of non-ductile bars for under-reinforced beams. For over-reinforced beams or columns with significant axial load, heavy and excessive confinement reinforcement is usually needed to achieve the ductility requirement;
Placing prestressed reinforcement in layers and design the effective prestress in each layer so as to provide a step-by-step progressive failure with increasing deformation. This method relies on the progressive fracture of FRP reinforcement to avoid sudden complete fracture of tension reinforcement;
Using partially prestressed concrete where prestressed FRP tendons are combined with conventional steel reinforcement to allow sufficient flexibility to achieve better ductility;
Using unbonded tendons so that more deformation can be achieved on the tension side as the deformation of the tendons over the whole unbonded length can be utilized. However, this implies the use of perfect anchorages that can sustain fatigue loading. Furthermore, external tendons can be very vulnerable to vandalism, and should they fail they will release an enormous amount of elastic energy that can be devastating;
Designing the interface between the FRP reinforcement and the concrete so that a bond failure is triggered when the stress in the tendons reaches a threshold level, thus changing a bonded tendon configuration to an unbonded tendon configuration; and
Designing the cross-section of a member to proportionate the reinforcement in order to take the advantage of the full strain capacity of concrete simultaneously with that of the reinforcement.
The success of such methods will vary depending on the specific application. However they are often considered either too complicated, too time consuming, overly expensive, or not very effective (i.e. limited increase in ductility).
Curvature, and hence flexural deformation, are due to tensile and compression straining at a cross-section. When tension yielding/deformation is unavailable, another avenue of achieving ductility/deformability is by compression yielding/deformation. In principle, all the methods of achieving flexural ductility/deformability of RC members must fall into these two categories.
It would be desirable to produce improved or alternative flexural members that overcame the problems associated with flexural members in the prior art.